The invention relates to load balancing in parallel database systems (e.g. a shared nothing database system).
A parallel database system (DBS) employing a shared-nothing architecture distributes the database among several host processors that run the DBS. Conventionally, such a system is implemented by assigning to each host processor its own set of one or more storage devices, e.g. disk assemblies. Each host processor has exclusive access to its own set of storage devices and it cannot access the storage device of another host.
If a portion of the database that is controlled by a specific host processor is accessed much more frequently than other portions of the database, the host processor that is responsible for that portion of the database becomes a bottleneck and the performance of the entire DBS degrades. Since only one host processor is able to process the accesses to the "hot" portion of the database, the other host processors are unable to contribute to completing the processing. Thus, the other processors go partially idle while the busy host processor completes its work. This results in a very inefficient use of resources and poorer performance than what would be possible if all processors could equally contribute to completing the processing.
One solution to this problem has been to rebalance the DBS by redistributing the "hot" portions of the database among the other hosts processors. Thus, the other host processors will be able to contribute to processing the requests to the hot portions of the database and processing efficiency can be improved significantly. The redistribution is typically accomplished by copying portions of the database from host to host, an operation which is generally very resource and time intensive.